The present invention relates generally to data encoding and transmission for telecommunications applications, and more specifically to serial encoding to permit reliable and efficient communication of digital data across a fiber optic cable.
In recent years there has been a desire to transmit data across fiber optic cables at ever increasing speed and over longer distances. Current efforts in the industry are targeted at data transmission rates of 40 Gb/s in a single optical channel over hundreds or thousands of kilometers. The demands of such operating frequencies are driving a need for greater performance from system components, such as optical transmitters. Additionally, because of the relatively high prices of components capable of operating at such frequencies, there is a need to realize cost savings where possible so that the prices of new systems can be set sufficiently low to attract customers. Additionally, because of the wide variety of network installations among different communications providers and the emerging nature of many of the formal and de-facto operating standards, it is desirable that system components be sufficiently flexible so as to be usable in a variety of configurations that might be required in such installations over the expected component lifetime.
Among the variations in network installations are the use of different modulation techniques. Non Return-to-Zero (NRZ) modulation is in widespread use and offers relatively simple system design. Binary Phase Shift Key (BPSK) modulation provides power-efficiency gain over NRZ at the expense of more complex transmitter and receiver design. A variant of BPSK is Differential Phase Shift Key (DPSK) modulation in which the baseband binary data is differentially encoded before optical phase modulation. Return-to-Zero (RZ) modulation has beneficial properties in combating distortions seen in fiber optic cables at longer distances. In RZ modulation, the light is turned on and then off to represent one state of a bit, and the light is left off to represent the other state of a bit. Variants of BPSK and DPSK modulation are Return-to-Zero Binary Phase Shift Key (RZ-BPSK) and Return-to-Zero Differential Phase Shift Key (RZ-DPSK) modulation, in which the light is turned on with one phase and then off to represent one binary state, and the light is turned on with another phase and then off to represent the other binary state.
In systems employing any of these modulation formats, the problem of accurately recovering the transmitted data from the received optical signal becomes more difficult as transmission speed and/or distance increases. Generating a very high-quality optical signal at the transmitter becomes more critical. Among the many features of an optical transmitter that contribute to the quality of the transmitted optical signal are the characteristics of the optical modulator. It is important, for example, that the modulator be accurately and stably biased to yield an accurate modulated signal with minimum noise. While there are known techniques for biasing optical modulators that have achieved good operational results, better performance is needed at current and future transmission rates and distances. In systems employing RZ, RZ-BPSK and RZ-DPSK modulation in particular, it is also important to obtain precise, low-jitter phase alignment between a modulating data stream and an RZ pulse stream that are combined to produce the modulated optical signal for transmission.